1. Field of the Invention
This invention relates to binder compositons based on aqueous alkali metal silicate solutions, i.e. waterglass solutions, containing organic polymers. Binders of this type may be used, for example, for waterglass primers, paints, plasters, coatings, cements and adhesives.
2. Statement of the Prior Art
Paints, plasters, adhesives and coatings based on waterglass solutions are relatively brittle and often not sufficiently waterproof after drying. Further disadvantages and problems of waterglass-based compositions of this type are that fillers and pigments cannot always be satisfactorily dispersed in pure waterglass solutions, and that the rheological properties of compositions of the type in question cannot be optimally adjusted as required for a number of applications.
There have been a number of previous attempts to optimize desirable properties of binders so that, along with adequate thermal and fire resistance, the various remaining surface properties are improved. Such improvements apply in particular to the rheological properties, dispersion behavior, adhesion, silicification and water resistance, coupled with a reduction in the brittleness of the dried films. However, it has been found that measures which improve some properties have precisely the opposite effect on others. For example, it is possible by increasing the molar ratio of SiO.sub.2 to Me.sub.2 O (Me=alkali metal) of the waterglass solution to increase the water resistance of the dried films. Unfortunately, this measure also increases the brittleness of the films or coatings.
German patent application No. 27 29 194, and corresponding British Patent Specification Nos. 2,001,333 and 2,058,818 describe adhesives and/or coating compositions for mineral fibers based on alkali silicate solutions which contain a material with more than 20% of the mineral kaolinite and a water-soluble, alkalistable organic polymer. The particular organic polymer disclosed is polymethacrylate. Adhesives such as these are said to be distinguished by favorable storage stability and also by the fact that the bindings and coatings obtained do not become brittle and are highly water-resistant.
German patent application No. 15 19 250 relates to a process for the production of stable, water-dispersible silicate-based powder mixtures using alkali silicate and fillers. In this process, an alkali silicate is mixed with an organic polymer, disclosed as an alkali (particularly sodium) polyacrylate and/or polygalactomannitol, and with a reactive or inert filler and the resulting mixture is dried. The powders obtained may be dispersed in water and may be used in that form as silicate paints and coating compounds. These dispersions are also said to be distinguished by favorable stability in storage and to give coatings characterized by high abrasion resistance, bond strength and water resistance.
It has been found that although some of the performance properties of the waterglass binders under discussion, for example their dispersion behavior or their rheological properties, can be improved by means of polylacrylates or polymethacrylates (chosen from the large number of different, commercially available products), other properties are always adversely affected at the same time.
For example, the dispersion behavior of waterglass solutions can be improved by the addition of polyacrylates or methacrylates containing a large number of carboxyl groups. Unfortunately, this addition gives rise to the disadvantage that, in acidic form, solid long-chain resins of the type in question cannot be properly incorporated, if at all, in waterglass solutions.
By contrast, the thixotropic properties of waterglass paints, plasters, coatings or adhesives can be improved by the addition of long-chain, crosslinked resins having a high acrylate content. Resins of this type are normally prepared and marketed in the form of aqueous dispersions. Dispersions such as these may be homogeneously stirred into waterglass solutions, even in acidic form, after dilution with water. However, the wetting agents used in the preparation of the dispersions, even if present in small quantities, reduce the water resistance of the waterglass solutions and cause them to foam, which in turn causes the formation of craters in the paints and coatings obtained therefrom.
Generally, it has been found that the addition of acrylate or methacrylate polymers does not sufficiently reduce the brittleness of the waterglass products. Additionally, some resins even increase the tendency towards cracking of the coatings obtained therewith.
U.S. Pat. No. 3,345,302 describes a formulation based on soluble silicates which consists of a solid, acidic component and of a solid, water-soluble silicate. The individual particles of the acidic component are coated (preferably sprayed) with a film-forming copolymer (which is insoluble in water at a pH of less than 8) of maleic acid anhydride and alkyl vinyl ethers or olefins of which only 50 to 70% is esterified with an alcohol. When this formulation is mixed with water whose pH is 8 or higher, the copolymer dissolves and releases the acidic component which, in turn, neutralizes the silicate now also dissolved and precipitates it in the form of silica gel. The solid formulations show high stability in storage and, after dissolution in water, are used as binders for refractory materials.
However, clear, alkaline aqueous solutions based on waterglass of the type which the present invention seeks to provide cannot be obtained in this way because the partial esters (i.e., esters in which at least one hydroxyl remains unesterified) of the copolymers described in U.S. Pat. No. 3,345,302 are too hydrophobic. This means that partial esters such as these cannot be incorporated in waterglass solutions, but instead always float on the solution.